The invention relates to a dispersive multilayer mirror comprising several individual layers applied to a carrier substrate and adjoining each other via parallel, plane interfaces and having different optical constants and different thicknesses. Such a mirror can be employed in laser devices so as to produce a given—negative or positive—group delay dispersion.
Furthermore, the invention relates to a method of producing such a multilayer mirror.
Ultrashort laser pulses (having pulse durations in the picosecond and femtosecond range) have a broad spectrum in the frequency range. Pulses with spectra which span an entire optic octave (between 500 nm and 1000 nm) have been demonstrated, and sources yielding pulses with a spectral width of approximately 200 nm (centered at 800 nm), are already commercially available. To form a short pulse in the time range, the frequency components of broad-band signals must also coincide. Because of the dependency of the refraction index on the wave length (also called “dispersion”), different components of the spectrum are differently delayed when passing through a dense optic medium. To describe this effect in terms of quantity, the group delay dispersion, or GDD in short, has been introduced as the second derivation of the spectral phase with respect to the angular frequency. The duration of a laser pulse remains unchanged when passing an optical system if the resultant GDD of the system is zero. If the system has a GDD≠0, the duration of the pulse at the exit from the optic system will have a different value than at its entry. To counteract these pulse changes, the GDD of the optical system must be compensated, i.e. a GDD with the same amount, yet a different preceding sign must be introduced. Various optical components have already been developed for carrying out this dispersion compensation, such as, e.g., prism pairs, gridpairs and dispersive mirrors (cf. e.g., U.S. Pat. No. 5,734,503 A and R. Szipocs et al., “Chirped multilayer coatings for broadband dispersion control in femtosecond lasers”, Optical Letters 1994, vol. 19, pp. 201–203, or WO 00/11501 A). On account of their great band width, the user friendliness and compactness, dispersive multilayer mirrors (so-called chirped mirrors, CMs) are used more and more frequently both for scientific and also for industrial applications.
During the reflection on a CM mirror, the different wave length components of the laser beam penetrate the layers of the mirror to different-depths before they are reflected. In this manner, the different frequency components are delayed differently long, corresponding to the respective depth of penetration. Since many optical components have a positive GDD, in most instances a negative GDD is required for the GDD compensation. To achieve a negative GDD, the short-wave wave packets are reflected in the upper layers of the CM mirror, while the long-wave portions enter more deeply into the mirror before they are reflected. In this manner, the long-wave frequency components are temporally delayed relative to the short-wave components, leading to the desired negative GDD. However, there are also applications in which a positive GDD is desired for compensation purposes.
One problem with these CM mirrors and, quite generally, with comparable multilayer mirrors consists in that at the interface of the uppermost layer relative to the environment, i.e. at the front face where the radiation impacts, a reflection that is largely independent of the wave length occurs (e.g. in the order of 3%). As a consequence, interferences occur between beams which are reflected at this front face, and beams which are reflected at a deeper point within the multilayer structure of the mirror, these interference effects possibly causing a distortion of the reflection ability and, above all, a marked distortion of the phase and dispersion characteristics of the mirror. To at least partially counteract this effect, it has already been suggested (cf. F. X. Kärntner et al., “Design and fabrication of double-chirped mirrors”, 1997, Opt. Lett. 22, 831; and G. Tempea et al., “Dispersion control over 150 THz with chirped dielectric mirrors”, 1998, IEEE JSTQE 4, 193, respectively) to apply an anti-reflective coating or a narrow-band suppression filter at the front face, i.e. at the interface to the ambiance (air, as a rule). To effectively suppress interfering resonances, the reflection at the front face should be in the order of merely 10–4%. Anti-reflection layers and suppression filters are, however, capable of approximating such properties over a very limited band width. Accordingly, dispersive multilayer mirrors in the past could be operated at 800 nm radiation over band widths of 150–160 THZ only. Moreover, a total suppression of the resonance interference effects is not even possible over such a band width, and the dispersion curve often shows marked fluctuations.
These interference effects which are caused by beams reflected at the front surface of the mirror can as such be effectively avoided, i.e. by means of a so-called TFI-mirror (TFI—tilted front interface), cf. the older, not pre-published WO 01/42821 A1): If the front face of the mirror is slightly “tilted” relative to the other interfaces, the beam which is reflected at this front face will propagate in another direction than the useful beam reflected by the mirror proper, so that it can no longer interfere with the latter in this far field. With this design, the band width of the dispersive mirrors can be increased by up to an optic octave. Although the structure of a TFI mirror in principle is simple, the production of such components does, however, pose several technological problems. In most instances, a TFI mirror must introduce a negative GDD, which can be achieved by means of a dispersive layer arrangement as described above; the wedge-shaped front layer, however, introduces a positive GDD which reduces the negative contribution of the mirror layers proper. In order not to substantially negatively affect the net dispersion of the mirror, the wedge-shaped layer should be as thin as possible. However, this wedge-shaped layer cannot have an arbitrary thinness because the wedge angle must have a certain minimum value so as to ensure an effective separation of the two beams. The ideal parameter of this layer, taking into consideration the above-indicated aspects, are: a wedge angle of H1°, and a thickness of approximately 20 μm to 50 μm at the thinnest edge. Such a thick layer cannot be produced by means of conventional coating methods (such as electron beam vapor deposition or magnetron sputtering). Therefore, there exist only the following two possibilities: (1) the uppermost wedge-shaped layer on the side on which the beam impacts is made of a thin, wedge-shaped platelet as carrier substrate, on which the other layers (the dispersive individual layers and an anti-reflection coating) are applied by a coating method; (2) the individual layers with the parallel interfaces are applied to a conventional thick optic carrier substrate by a coating method, and on these individual layers, subsequently a thin wedge-shaped platelet is applied or produced by means of a special technological method different from a coating method. A disadvantage of method (1) consists in that the surface quality of the TFI mirror will be negatively affected by the tensions in the layers. Since the carrier substrate must be thin, the slightest tensions (which are unavoidable in a vapor deposition or sputtering coating) will lead to an arching or irregularity of the thin wedge-shaped substrate. The second possible way does not harbor this problem because the carrier substrate may have any thickness desired. In this instance it must, however, be ensured that an impedance adaptation is realized between the individual layers of the mirror and the wedge-shaped platelet so as to avoid the previously described interference effects. For this purpose, the use of an index adaptation fluid (also IMF index matching fluid) has been suggested (cf. the older, not pre-published WO 01/42821 A). In this manner, a nearly perfect impedance adaptation is achieved, because the commercially available IMFs are capable of reproducing the refractive index of glass with a precision of 10-4. Yet also in this instance, the surface quality of the mirror is not satisfactory, because there is no tight connection between the thin wedge-shaped platelet and the multilayer structure of the multilayer mirror; due to the slight thickness, the quasi-free standing wedge-shaped platelet cannot have a surface quality of λ/10 (as is common in laser technology).
For the sake of completeness, it should be pointed out that wedge-shaped optic multilayer components have already been suggested for most varying applications, such as, e.g., for suppression filters, interference light filters, wave-length selective mirrors, beam dividers or the like, wherein, in particular, comparatively thick wedge platelets are used as substrate for multilayer structures (GB 1,305,700 A; EP 416 105 A; EP 533 362 A; U.S. Pat. No. 4,284,323 A; GB 2,054,195 A=DE 3 026 370 A). Thus, these are components different from dispersive multilayer mirrors which shall cause a certain group delay dispersion wherein, moreover, the usual coating techniques are used with the afore-mentioned disadvantages as regards laser quality etc.